Single-walled carbon nanotubes (SWCNTs) are key building blocks for nanoscale science and technology due to their interesting physical and chemical properties. SWCNTs are particularly promising for high speed and low power semiconductor electronics. A challenge, however, is the hierarchical organization of these building blocks into organized assemblies and, ultimately, useful devices. Ordered structures are necessary, as random network SWCNT thin films result in sub-optimal electronic properties including reduced channel conductance and mobility. Numerous techniques for aligning SWCNTs have been explored to solve this shortcoming and achieve higher conductance and mobility. These approaches can be divided into two main categories: (a) direct growth via chemical vapor deposition and arc-discharge, and (b) post synthetic assembly. In the case of direct growth, both metallic and semiconducting SWCNTs are produced. In this case, the performance of SWCNT field effect transistors (FETs) is limited by the metallic SWCNTs (m-SWCNTs), thus motivating attempts to purify semiconducting SWCNT (s-SWCNT) samples with homogeneous electronic properties.
A variety of post-synthetic sorting methods have been developed to separate m- and s-SWCNTs according to their specific physical and electronic structures, which are usually implemented in aqueous or organic solutions. In order to take advantage of the high purity of s-SWCNTs that can be produced by these solution-based sorting approaches in semiconductor electronic devices, solution-based methods for assembling and aligning s-SWCNTs, such as evaporation-driven self-assembly, blown-bubble assembly, gas flow self-assembly, spin-coating, Langmuir-Blodgett and -Shafer methods, contact-printing assembly, and AC electrophoresis, have been developed. (See, Shastry, T. A.; Seo, J. W.; Lopez, J. J.; Arnold, H. N.; Kelter, J. Z.; Sangwan, V. K.; Lauhon, L. J.; Marks, T. J.; Hersam, M. C. Large-area, electronically monodisperse, aligned single-walled carbon nanotube thin films fabricated by evaporation-driven self-assembly. Small 2013, 9, 45-51; Druzhinina, T.; Hoeppener, S.; Schubert, U.S. Strategies for Post-Synthesis Alignment and Immobilization of Carbon Nanotubes. Adv. Mater. 2011, 23, 953-970; Yu, G.; Cao, A.; Lieber, C. M. Large-area blown bubble films of aligned nanowires and carbon nanotubes. Nat. Nanotechnol. 2007, 2, 372-7; Wu, J.; Jiao, L.; Antaris, A.; Choi, C. L.; Xie, L.; Wu, Y.; Diao, S.; Chen, C.; Chen, Y.; Dai, H. Self-Assembly of Semiconducting Single-Walled Carbon Nanotubes into Dense, Aligned Rafts. Small 2013, 9, 4142; LeMieux, M. C.; Roberts, M.; Barman, S.; Jin, Y. W.; Kim, J. M.; Bao, Z. Self-sorted, aligned nanotube networks for thin-film transistors. Science 2008, 321, 101-4; Cao, Q.; Han, S. J.; Tulevski, G. S.; Zhu, Y.; Lu, D. D.; Haensch, W. Arrays of single-walled carbon nanotubes with full surface coverage for high-performance electronics. Nat. Nanotechnol. 2013, 8, 180-6; Jia, L.; Zhang, Y.; Li, J.; You, C.; Xie, E. Aligned single-walled carbon nanotubes by Langmuir-Blodgett technique. J. Appl. Phys. 2008, 104, 074318; Liu, H.; Takagi, D.; Chiashi, S.; Homma, Y. Transfer and alignment of random single-walled carbon nanotube films by contact printing. ACS Nano 2010, 4, 933-8 and Shekhar, S.; Stokes, P.; Khondaker, S. I. Ultrahigh density alignment of carbon nanotube arrays by dielectrophoresis. ACS Nano 2011, 5, 1739-46.) While each of these methods has its strengths, new methods are still needed to improve the fidelity of s-SWCNT assembly and alignment in order to enable the fabrication of practical s-SWCNT-based electronic devices.